SLH 350 Test 2 PPTs 4-6

Lecture 15

ECOG and Invasive Electrophysiology

Low spatial resolution

We can see overall brain structure (at different scales), but not single neurons

Low spatial resolution looks like…

a blurry photo

High spatial resolution

can see single neurons or small groups of neurons in specific brain areas

HIgh spatial resolution looks like…

a high-def camera/TV

Some methods have very good spatial resolution (you can see individual neurons), but

you can only see a few neurons at a time.

Some methods have pretty good (medium to high) spatial resolution, but

you can see the whole brain

Low temporal resolution

• We can only get a snapshot of brain processes every few seconds, minutes, or hours

• Like timelapse, only one photo every X seconds

High temporal resolution

• We can see changes that are milliseconds apart (like in the action potential!).

• Like high-speed video, many frames per second.

Spatial and temporal resolution are…

Relative! There is no one best method for everything

Invasive EP/NP (Electrophysiology/Neurophysiology)

Invasively record from neurons using an electrode (generally made from wire or from glass).

Invasive EP/NP requires…

some kind of surgery

Invasive EP/NP is usually done in animal models, or in very limited circumstances

on people (for example, in tissue that will later be resected in a surgery).

Invasive EP/NP records…

action potentials

Invasive EP/NP measures

either cell voltage or currents flowing through the cell.

Invasive single-unit electrophysiology

measures one neuron

Invasive multi-unit electrophysiology

Measures approximately tens of neurons at a time.

Invasive electrophysiology has _____ spatial resolution and _____ temporal resolution

high, high

glass electrodes from electrophysiology

These glass electrodes would be used for intracellular recordings (e.g., "patch clamp").

Whole-Cell Intracellular Recordings (Single Unit)

• The electrode is filled with some solution (usually with the same concentrations as intracellular fluid).

• The electrode is advanced into the brain tissue until contact is made with a cell.

• The researcher applies suction to suck up some of the plasma membrane into the pipette, then records action potentials from a single neuron.

Electrodes for Extracellular (Multi-Unit) Electrophysiology

• Can also record from the outside of the cell with a metal electrode:

  • Sometimes one piece of metal.

  • Sometimes four wires twisted together.

• Measures multiple neurons at once.

Single neuron recordings in humans

Neuropixels probes

Electrocorticography (ECoG)

A grid of electrodes is placed on the surface of the brain to localize seizures in people with drug-resistant epilepsy.

Each electrode signal in ECoG records from hundreds to thousands of neurons near that electrode. This signal is called the

local field potential (LFP)

ECoG has _______ spatial resolution and ______ temporal resolution

medium-high, high

ECoG comparison to multi- and single-unit recordings

• ECoG has similar temporal resolution to single- and multi-unit recordings.

• ECoG has worse spatial resolution.

ECoG ethical disclaimer

ECoG is done for clinical purposes only, with patient consent research studies have been conducted.

• Research studies depend on electrode coverage.

Some Experiments on Speech/Language Function

• "Brain-to-text": Many groups are working on using ECoG, Utah arrays, or noninvasive EEG/fNIRS to decode thoughts in people who cannot speak.

• Recordings from the superior temporal gyrus during speech listening — how does the brain represent speech features?

Lecture 16

EEG and MEG

What does EEG stand for?

Electroencephalography

What Does EEG Measure?

Electrical activity of neurons produces currents spreading through the head.

Presynaptic currents (axon, axon potential) are generally

not measurable by EEG

Post synaptic currents (dendrites, excitatory post-synaptic potential (EPSP)) are generally

measurable by EEG

The post-synaptic currents reach the surface of the scalp, in the form of

voltage changes and magnetic fields

Measuring voltage changes at the scalp is called…

electroencephalography (EEG)

The voltage changes detected by EEG are generated by

approx 500,000 to 1 million neurons

Recording EEG

metal discs or cup electrodes

When recording EEG, the discs (or cup electrodes) may be applied directly on the head or…

as a net, or with a cap

When recording EEG, the tech may use conductive gel or saline to

get better conductivity through the scalp

We cannot read a ____ EEG, it must be filtered and any _____ removed

raw, noise

Is EEG invasive?

no, it is non-invasive

Is EEG cost effective?

yes

EEG has ____ temporal resolution

hihg, you can look at things that happen quickly (milliseconds apart)

EEG is used clinically for…

sleep studies, epilepsy studies, hearing screenings (evoked potentials/auditory brainstem response)

EEG is used for research in

language, vision, cognition, etc.

EEG has _____ temporal resolution, but _____ spatial resolution

high, low

EEG has great timing, but poor

localization (where in brain)

EEG evoked potentials are used in

Universal newborn hearing screening (ABR = auditory brainstem response).

EEG can be used to examine specific components related to

early sensory processing versus later "cognitive" components.

EEG can be used clinically to monitor

seizure activity in epilepsy, as well as monitoring normal function

Event Related Potentials (ERP)

One method of EEG analysis, also called evoked potential

ERP event related =

happens in response to a stimulus (visual, auditory, motor, etc.)

In ERPs, the stimulus is repeated a bunch of times, then

the response to the stimulus is measured and recorded

ERPs can do this for any stimuli, like

tones, speech sounds, faces, etc.

Visual ERPs Are Larger in People With Hearing Loss

• Larger responses to visual stimuli for the hearing-loss group (red) versus controls (blue).

• Participants had adult-onset mild-to-moderate hearing loss.

MEG stands for

Magnetoencephalography

MEG measures

tiny magnetic fields generated by neurons' electrical currents.

MEG uses SQUIDs

superconducting quantum interference devices

MEG has ______ temporal resolution, and _______ spatial resolution

high (milliseconds), medium-high

MEG is great at capturing…

eural activity in the sulci, but not the gyri, due to the physics of the magnetic field.

MEG has better spatial resolution than EEG because

magnetic fields are not blurred by the skull.

MEG and EEG have similar

temporal resolution

MEG is sensitive to activity in

sulci, but less so in gyri.

True or false: MEG systems are very expensive and must be housed in special rooms.

True

SQUIDs must be kept cool with

liquid helium, which is expensive and rare, although new tech is being developed

Clinical Uses of MEG

1. Motor mapping before surgery

2. sensory mapping before surgery

3. language mapping before surgery

Research uses of MEG

Many! (like EEG) generally questions that need high temp res and benefit from higher spatial res

Analyses That Work for Both EEG and MEG

• Event-related potentials (ERPs).

• Spectral analysis (examining EEG/MEG in particular frequency bands associated with neural oscillations).

• Time-frequency analysis.

Spectral Analysis

examining EEG/MEG in particular frequency bands associated with neural oscillations

Spectral analysis filters…

EEG or MEG into different neural oscillation bands

In Spectral Analysis, different bands are associated with

different cognitive tasks and states of arousal

Frequency bands in Spectral analysis

delta, theta, alpha, gamma

Delta bands

0-4 Hz, slow fluctuations, approx. word level rate

Theta bands

4-8 Hz, syllabic rate

Alpha bands

8-12 Hz, associated with attention

Gamma Bands

30-100+ Hz

Time-Frequency Analysis looks at

timing and spectral content at the same time

Time-Frequency Analysis may see an activity

in a task localized to a certain band

Time-Frequency Analysis example

• English and Mandarin speakers listened to Mandarin Chinese sentences played at a regular rate of 4 words per second (4 Hz).

• Each two-word pair was a noun phrase or verb phrase.

• Each four-word utterance was a sentence.

• MEG was recorded.

Time-Frequency Analysis experiment findings

The brain tracked sentence structure only when participants understood the language.

EEG or MEG? A researcher wants to study speech-sound discrimination in awake, active six-month-old infants in a relatively naturalistic room setting, on a limited research budget.

EEG

EEG or MEG? A researcher needs to precisely localize the cortical source of an auditory response to within a few millimeters to distinguish activity in neighboring auditory regions.

MEG

EEG or MEG? A clinical team wants a bedside-compatible tool to monitor a patient's brain activity continuously overnight.

EEG

EEG or MEG? A research team is building a large, multisite study and needs equipment that is relatively inexpensive and easy to transport between testing locations.

EEG

Lecture 17

FMRI and FNIRS